243 



Except, perhaps, for providing initially a reasonable margin of in- 

 creased swept depth over areas that must be drilled and blasted, the 

 sensible procedm-e in nearly every case is to lay out the work from 

 existing low water datums, and to determine any required changes in 

 these datums by direct observation after the improvement has been 

 made. Only in most exceptional cases will doubt or controversy over 

 the consequences of the effect of enlargements or contractions justify 

 a prior computation. In the following paragraphs^ an outline is 

 suggested of computations which should afford results in which some 

 confidence may be placed when the flow is essentially tidal. If the 

 fresh-water flow dominates the currents and tides, recourse to a 

 hydraulic laboratory might be necessary. 



469. Computation of changes in mean low water datum. — If the tides 

 are of the semidiurnal type and if the ordinary fresh-water flow is 

 small in comparison with the tidal flow, or if the adopted mean low 

 water datum is established from the tides during periods in which the 

 fresh-water flow is inconsiderable, the changes in mean tidal range at 

 stations along an estuary, resulting from proposed enlargements or 

 contractions of the channel, should be substantially proportional to 

 the changes in the primary tides corresponding to the mean tidal 

 fluctuations. The mean primary tides before improvement may be 

 determined from the tide records, and those after improvement 

 computed by the formulas developed in chapter VIII, paragraph 422, 

 with coefficients derived from the corresponding primary currents 

 before improvement. 



470. Primary tides and heads before improvement. — ^To afford the 

 requisite data for the computations, tide gages must be established at 

 suitable stations from the head of tide to a point at which the cross 

 section of the estuary is so large that the eft'ect of the improvement 

 upon the currents will become too small to be considered. These 

 stations should be placed at the more marked changes in the cross 

 section of the estuary, and at such distances from each other that the 

 water surface between them will not depart materially from a plane 

 surface. They establish the ends of the subsections into which the 

 channel is to be divided. From the tide records during a period of 15, 

 or preferably 29, consecutive days of low upland flow, average tide 

 curves are prepared for each station as described in paragraph 304, 

 and the corresponding primary tides computed from the heights at 

 successive lunar hours as explained in paragraph 360. The coordi- 

 nate amplitudes of the primary tides are then determined. Their 

 differences between the successive stations give the coordinate 

 amplitudes of the primary heads between the stations, from which 

 the amplitudes, H, and initial phases, H°, of the heads in the sub- 

 sections are determined. 



